This invention relates generally to differential mechanisms and more specifically to torque biased differential mechanisms which are used in four wheel drive vehicles.
The drive train for four wheel drive vehicles commonly include an auxiliary transmission or transfer case which has a drive input which is driven by the main transmission and front and rear drive outputs for driving the front and rear drive axles respectively. These auxiliary transmissions or transfer cases may include a differential mechanism, commonly referred to as an inter-axle differential, to accommodate the differences in front and rear wheel speeds, particularly in cornering maneuvers.
Inter-axle differentials which split the torque evenly, or at some fixed ratio, between the front and rear outputs are known as "open" differentials. A known drawback of these "open" differentials is that the vehicle wheels will not produce enough tractive force to drive the vehicle if any one of the four wheels is on a slippery surface. Consequently, inter-axle differentials are often of torque biased type which insure that a useful amount of the input torque is directed to the drive output for the wheels which can produce sufficient traction.
Known torque biasing devices include manually applied positive (dog) clutches, friction plate or cone clutches which are either spring or hydraulically loaded, speed responsive locking clutches, viscous clutches or couplings and fluid couplings all of which couple the two outputs of the differential mechanism in one way or another.
Of these known torque biasing devices, the fluid coupling has several very desirable operating characteristics because it produces a variable torque bias which is proportional to the square of the speed difference between the two outputs of the differential mechanism. On the other hand, the variable torque produced by the fluid coupling is also directly proportional to the diameter of the fluid coupling and thus the use of the fluid coupling as a torque biasing device is not feasible for automotive applications because of the space requirements. This is particularly so in the case of inter-axle differentials for four wheel drive passenger type vehicles.